The present invention relates in general to pumping devices, and more specifically, to improved blood pumps with levitated impellers and methods for their control.
Mechanical circulatory support (MCS) devices are commonly used for treating patients with heart failure. One exemplary type of MCS device is a centrifugal flow blood pump. Many types of circulatory support devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. Examples of LVAD systems are the DuraHeart® LVAS system made by Terumo Heart, Inc. of Ann Arbor, Mich. and the HeartMate II™ and HeartMate III™ systems made by Thoratec Corporation of Pleasanton, Calif. These systems typically employ a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller is formed as the rotor of the electric motor and rotated by the rotating magnetic field from a multiphase stator such as a brushless DC motor (BLDC). The impeller is rotated to provide sufficient blood flow through the pump to the patient's systemic circulation.
Early LVAD systems utilized mechanical bearings such as ball-and-cup bearings. More recent LVADs employ non-contact bearings which levitate the impeller using hydrodynamic and/or magnetic forces. In one example, the impeller is levitated by the combination of hydrodynamic and passive magnetic forces.
There is a trend for making blood pumps more miniaturized to treat a broader patient population, more reliable, and with improved outcomes. To follow this trend, contactless impeller suspension technology has been developed in several pump designs. The principle of this technology is to levitate the pump impeller using one or a combination of forces from electromagnets, hydrodynamics, and permanent magnets. In the meanwhile, the pump should be hemocompatible to minimize the blood cell damage and blood clot formation. To that end, the bearing gap between the levitated impeller and the pump housing becomes an important factor. A small gap may lead to the high probability of the thrombus formation in the bearing or to elevated hemolysis due to excessive shear stress. Likewise, a large gap can compromise the hydrodynamic bearing performance and the pump efficiency.
One pump design utilizing active magnetic bearings achieves the desired bearing gap by levitating the impeller using magnetic fields generated by electromagnetic coils. However, in such a design there is the need for a separate bearing control system that includes the position sensors and electromagnetic coils to control the impeller position.
Another pump design levitates the impeller using hydrodynamic thrust bearings alone or combined with passive magnetic bearings. However, such a design usually requires a small bearing gap to provide sufficient hydrodynamic bearing stiffness to maintain impeller levitation and prevent contacts between impeller and the pump housing. Such a small gap may result in an insufficient washout and vulnerability to blood clotting thus compromising hemocompatibility.
Pumps utilizing hydrodynamic bearings to suspend the impeller are generally designed to maintain a generally constant gap through all operating conditions. A drawback of such designs is that the impeller starts to tilt when the pump flow rate increases. This impeller position shift under low pressure conditions across the narrow gap creates blood flow stasis, which in turn leads to thrombus formation on the bearing surfaces. One solution to solve the problem is to add a passive magnetic bearing to try to maintain a stable gap. However, the magnetic bearing solution increases the size of the pump and complexity of the system.
What is needed is a pump that addresses these and other problems of known designs. What is needed is a pump with a relatively small form factor and improved outcomes. What is needed is a pump that employs a simple bearing system and enhances blood flow gaps to reduce the risk of thrombus. What is needed is a solution to enhance the bearing gap to achieve adequate washout without increasing the complexity of the pump mechanical design or reducing the pump efficiency.